For high power laser systems, the development of efficient, large aperture amplifiers can significantly improve the performance to cost ratio, due to the high fixed costs and complexity associated with the usual array of small aperture chains of equivalent output power. At one extreme, the use of a large aperture amplifier may avoid the constraint of maximum beam intensity imposed by optical damage thresholds (normally 4-10 J/cm.sup.2 for the most easily damaged components), by distributing the beam to be amplified over a larger area. At the other extreme, one is limited at the upper end on size of an amplifier element by the amplified fluorescence losses within the element; this process limits the available gain coefficient by depletion of the population inversion of the amplifier medium.
In "Optimization of Nd:Glass Lasers with Phosphate-Laser Glass" J. M. McMahon, NRL Memorandum Report 3411 (November 1976) arrays of separate beams are spaced so closely together that flashlamps illuminating one solid state amplifier segment illuminate the segments in a separate adjacent laser amplifier. Only circular cross-sectional beams appear to be contemplated, and each beam is a separate laser beam run through a common pumping cavity with some flashlamps illuminating amplifier segments in adjacent beams.
In V. V. Alexandrov et al, "Experimental Studies on the Interaction of Laser Radiation with Plasma in Mishen-1 and Mishen-2 Devices", IAEA-CN-33/F3-2, p. 365 at p. 369 a three Nd:glass slab amplifier is disclosed with slabs 40.times.240.times.700 mm to amplify three well-collimated laser beams in parallel. The location of the flashlamps is not disclosed, but the flashlamps are termed closely packed. While in oral conversations with some of the Russian designers of the Mishen-1 and 2 devices it was disclosed that the flashlamps are positioned between the Nd:glass slabs, as well as about their exterior, but the inventor has no independent knowledge to affirm that or the arrangements and effectiveness of same. The flashlamps have been placed with their longitudinal axis running parallel to the laser beams and in other designs running perpendicular to the laser beams. Visiting American scientists have been shown the Mishen-1 apparatus. The Russian work shows only a single solid slab for a given amplification stage per laser beam, and the laser beams at the parallel amplifiers are considered separate beams of a multiarm laser.
Russian and American designers have given attention to reducing the number of flashlamps and power supplies for a large laser through grouping separate parallel laser beams in a single amplifier cavity with separate amplification media for each beam. Making a single large beam from a segmented amplifier was not addressed. Also problems remain for the Russian approach of using solid slabs for an entire amplification stage per beam in trying to attain uniform pumping, in weight of the amplification medium, and in time the amplification medium takes to cool after a laser pulse has passed through. The use of disc amplifiers greatly eases these restrictions; however, discs are only presently designed for the 30-45 cm aperture range at maximum. The largest disc amplifier aperture presently used is 20 cm on both Argus and Shiva at Lawrence Livermore Laboratory in Livermore, Calif. NRL has considered disc amplifiers for a clustered set of separate laser beams run through a flashlamp cavity, but NRL did not progress to the more compact rectangular discs or amplification of a large cross-sectional laser beam by a segmented amplifier.